Add multi-database data layer

Introduce aether-data-schema and driver-specific schema generation for Postgres, MySQL, and SQLite.

Split data backends, lifecycle, repositories, and gateway runtime integration across database drivers.

Verified with cargo fmt --all --check, cargo clippy --workspace --all-targets -- -D warnings, and cargo test --workspace.

crates/aether-data/src/lifecycle/mod.rs

@@ -0,0 +1,9 @@
+//! Database lifecycle workflows.
+//!
+//! Runtime request paths should not depend on this module directly except at
+//! process startup or explicit maintenance/export commands.
+
+pub mod backfill;
+pub(crate) mod bootstrap;
+pub mod export;
+pub mod migrate;